JP7-14850A discloses a heterojunction field effect transistor which includes an undoped GaAs layer and an undoped InGaAs layer as an active layer, and AlGaAs layers in which Si is partially added and which sandwich the active layer. JP10-56168A discloses a field effect transistor in which a difference in electron affinity around the interface between a lower carrier supply layer and a channel layer of the field effect transistor is larger than a difference in electron affinity around the interface between the channel layer and an upper carrier supply layer. JP11-354776A discloses an high electron mobility transistor (HEMT) using crystal stack in which a n-type AlGaAs carrier supply layer, an undoped AlGaAs spacer layer, an undoped GaAs channel layer, an undoped InGaAs channel layer, an undoped GaAs spacer layer, and an n-type GaAs channel layer are sequentially layered on a semi-insulating GaAs wafer.
JP2000-183334A discloses a heterojunction field effect transistor which includes buffer layers made of GaAs and AlGaAs, a lower electron supply layer made of n-type AlGaAs, a channel layer made of i-type InGaAs, an upper electron supply layer made of n-type AlGaAs, a schottky layer made of i-type AlGaAs, an ohmic contact layer made of n-type GaAs or the like, a gate electrode made of WSi, and a source electrode and drain electrode made of Au, Ge or Ni. In this transistor, the product Nt for the upper electron supply layer is about 1.4 times of the maximum sheet carrier concentration at the heterojunction interface, and the product Nt, wherein N is the doping concentration and t is the thickness of the layer, for the lower electron supply layer falls within a range of 1.0 to 2.0 times of the maximum sheet carrier concentration.
Patent Document 1: JP7-14850A
Patent Document 2: JP10-56168A
Patent Document 3: JP11-354776A
Patent Document 4: JP2000-183334A